Federal, state and local governmental bodies reacting to constituent pressures have instituted a series of laws and regulations aimed at protecting the public health and preventing the continued contamination of the environment. Heavy metals are generally defined as hazardous and, therefore, must be removed from natural waters and industrial effluent streams. Once removed from these streams, the heavy metals-containing waste has been containerized and then disposed of in government-sanctioned landfills. These special landfills are now being more closely monitored thereby forcing alternative methods of disposal of these solid heavy metal wastes. It is toward both the clean-up of these natural waters and effluent streams and the discontinued pollution of soil and ground waters that the invention of this method is aimed.
Progressively stricter regulatory criteria have forced industry to drastically reduce the residual metal content in wastewater discharges. Likewise, public water treatment agencies are being forced to maintain or improve the quality of public water supplies by removing trace amounts of man-made and naturally-occurring contaminants. Obviously, regulations pertaining to radioactive isotope-containing waters are among the most stringent and among the costliest with which to comply. Increased cost for the disposal of solid metal wastes also have forced industries and governmental agencies to examine present treatment techniques and to demand more efficient and cost effective alternatives to those currently available.
The ability of conventional water treatment methods to achieve the low levels of residual metals required by the higher standards for water purity in many cases is marginal. Recent legislation has made the disposal of sludge material extremely difficult and expensive, with no near term solution to the sludge disposal problem being apparent.
Because of these problems, industry and public health agencies in general, and the nuclear reaction segments in particular, have been forced to consider alternative methods for heavy metals removal from natural and wastewater streams. The major objectives of heavy metals removal methods from various waters are: ability to reduce residual metal contents to extremely low levels (ultimately to the parts-per-billion range or, in the case of radioactive isotopes, to pico-curies or parts-per-trillion range); production of a water supply suitable for public consumption; production of minimal amounts of sludge; economical operation; production of an effluent suitable for disposal or recycle to process operations; and ability for maximum retrofit into existing operations.
Some of these problems were addressed in an analysis of the processes used in treating drinking water for the removal of radioactive contaminants, and of the disposal of wastes generated by these processes in TREATMENT, WASTE MANAGEMENT AND COST FOR REMOVAL OF RADIOACTIVITY FROM DRINKING WATER, G. W. Reid, P. Lassovsky, and S. Hathaway, Health Physics, 48 (1985) pp. 671-694. The alternative processes, including ion exchange, reverse osmosis or electrodialysis, lime and lime-soda softening, greensand, manganese fiber, coagulation techniques and activated alumina, were evaluated in terms of cost, efficiency, reliability, process control and feasibility for the removal of uranium, radium and radon from water. Each of the alternative processes has disadvantages making necessary the continued search for a safe, effective method of radioactive metals removal with a minimum of waste product formation.
For instance, manganese dioxide, an effective absorber of many metal ions, was used to remove naturally-occurring radioactive radium from water supplies in Illinois and Iowa. On a laboratory scale, it was found that passing the radium-containing water through a vessel containing a manganese dioxide-impregnated fibrous filter media removes up to 90% of the radioactive radium. Also, this method did not require the backwashing or regeneration of the resin bed that is required in ion exchange methods, thus avoiding the liquid wastewater discharge disposal problem. However, the manganese dioxide-impregnated fiber method does have severe disadvantages including difficult preparation and handling of the impregnated fibers, the need for qualified operators, and poor practical performance since up to 50% of the loosely held manganese dioxide is washed out of the fiber during water treatment. These disadvantages illustrate why, to date, no practical, cost-effective, simple method is available for the removal of naturally-occurring radioisotopes from water supplies.
In Belgian Pat. No. 887,710, radionuclide-containing effluents from nuclear reactors are decontaminated by contacting the effluent with a solid inorganic non-radioactive material, followed by separation of the decontaminated liquid effluent from the solid or solid-liquid fraction containing the radionuclides. The inorganic non-radioactive material is usually a metal oxide, a spinel or a zeolite, and preferably is manganese dioxide. The inorganic non-radioactive material is discarded after contact with the radionuclide-containing effluent. A major disadvantage of this method is the large volume of solid or solid-liquid waste that is generated.
One of the more promising new alternative approaches that possesses the potential of fulfilling to a significant degree the desirable requirements for treating metal-bearing liquids is xanthate technology. A patent to John Hanway Jr. et al., U.S. Pat. No. 4,166,032, discloses the use of cellulose xanthate for heavy metals removal from wastewater streams. While cellulose xanthate is very effective for the removal of heavy metals from wastewater, the cellulose xanthate adds an amount of sludge equal to the dry weight of the cellulose xanthate added to the wastewater stream further increasing both the weight and volume of the sludge generated. Also, cellulose xanthate cannot be used successfully in a continuously flowing process wherein the removal material is held in a flow column and capable of periodic replacement.
In accordance with the present invention, it has been found that one or more water-insoluble carboxylated celluloses, such as an aluminum salt of carboxymethylcellulose, can remove heavy metals, and, in particular, radioactive heavy metal isotopes in new and unexpected proportions from liquids, such as nuclear fuel manufacturing wastewater streams, natural waters, and other wastewaters and nuclear-contaminated oils, leaving a substantially non-polluted solution or effluent capable of plant recycle or legal discharge.
It is known that insoluble forms of cellulose, such as carboxymethylcellulose, are effective in removing certain heavy metals such as Al, Cr, Sn, Pb, Fe, Cu, Ni and Zn from a wastewater, as disclosed in A SYSTEM OF ION-EXCHANGE CELLULOSES FOR THE PRODUCTION OF HIGH PURITY WATER, Horwath Zs, Journal of Chromatography, 102 (1974) pp. 409-412. However, such insoluble celluloses have not been used for removal of the radioactive isotopes of elements such as U, Cs, Sr, Ra, Ru, Rh, Np or Tc from waste streams. Further, such insoluble carboxylated celluloses have not been insolubilized in the presence of other solid heavy metal interactants, such as absorbers, adsorbers, reactants, or cation exchange materials to entrap the other heavy metal interactant within a water-penetrable water-insoluble carboxylated cellulose network, as accomplished in accordance with one embodiment of the present invention. As disclosed in the Horwath article, the insoluble carboxymethylcellulose is disposed in a column in a sandwich-type arrangement with other forms of ion-exchange celluloses and the wastewater passed through the column, with the ion exchange celluloses acting as a filtering media for absorption of the heavy metals therein.
U.S. Pat. No. 4,260,740, assigned to Pfizer, Inc., also discloses that insoluble carboxylated cellulose is useful as an ion exchange material for removal of heavy metals from an industrial effluent and for precious metal recovery. The process disclosed in U.S. Pat. No. 4,260,740 teaches a reaction of cellulose with polycarboxylic acids followed by a hydrolysis step in dilute alkali at a pH of 8 to 11 to bind each polycarboxylic acid moiety to the cellulose and thereby increase the ion exchange capacity towards heavy metal ions.
The removal of heavy metals, especially radioactive isotopes, from a liquid requires that concurrent consideration be given to disposing of the removed heavy metals. It is extremely advantageous to generate a low volume heavy metal-containing solid or sludge that may be safely and economically treated and disposed of. It has been found that the resulting radioactive bed from an insoluble form of carboxymethylcellulose and a heavy metal interactant, such as a transition metal oxide, can be treated easily using existing technology to produce small volume, radioactive ceramic fibers and spheres. The overall radioactive waste is thus reduced in volume by several factors, allowing for easier and less expensive disposal.
U.S. Pat. No. 4,537,818 teaches the manufacture of free-standing metal oxide films by absorbing cations such as U, Zr, Nd, Ce, Th, Pr, and Cr onto carboxymethylcellulose. The heavy metal-impregnated film first is heated in an inert atmosphere and then oxidized to form a metal oxide membrane useful as a nuclear acceleration target material.
In accordance with the present invention, heavy metals, including their radioactive isotopes, are removed from liquids to an unexpectedly high degree by contacting the liquid with an insoluble carboxylated cellulose, such as an insoluble salt of carboxymethylcellulose, and a heavy metal interactant, e.g., absorber, adsorber, reactant and/or ion exchange material, such as a transition metal oxide. The resultant radioactive heavy metal-containing mixture being converted to a non-leaching, ceramic-type mineral, that is suitable for safe disposal.